The present invention relates to apparatus for ascertaining the rheologic properties of flowable materials. More particularly, the invention relates to improvements in viscometers of the type wherein the material to be tested is subjected to shearing stresses and the test involves determination of the speed at which the material is deformed in response to such stresses. Still more particularly, the invention relates to improvements in viscometers of the type wherein a weighted measuring element (e.g., a cord, strip, band, rod or shaft) moves relative to a stationary measuring element and a sample of the material to be tested is introduced between the stationary and mobile measuring elements.
In viscometers of the just outlined character, the mobile measuring element (i.e., the aforementioned rod, shaft, cord, band or strip) moves with some clearance through a circular or slit-shaped hole of the stationary measuring element and a sample of the material to be tested is introduced into such clearance. As the mobile element moves relative to the stationary element, the sample is subjected to shearing stresses and the velocity of the mobile element is an indicator of the viscosity of the sample.
In order to allow for convenient and more accurate plotting of a curve which denotes the viscosity of tested material, it is necessary to carry out several tests, preferably at least four successive tests during each of which the sample is subjected to a different shearing stress. When the mobile measuring element is moved by resorting to weights, it is necessary to connect the mobile element with at least four different weights, one after the other, and to ascertain the speed of vertical downward or upward movement of the mobile element under the action of each of these different weights. As mentioned above, the speed of the mobile element relative to the stationary element is proportional to (or can be utilized for ascertainment of) the viscosity of tested material.
In presently known viscometers wherein a rod or shaft moves vertically with respect to a stationary cylindrical or annular measuring element and wherein the sample of a material to be tested is introduced into a narrow annular clearance between the internal surface of the cylindrical or annular element and the periphery of the mobile element, the testing operation is carried out as follows: The mobile element is connected with a first weight and is allowed to move downwardly at a speed which is dependent on the mass of the weight and on viscosity of the sample, the mobile element is then returned to its starting position and the first weight is thereupon removed and replaced with a heavier second weight, and so forth. The just described conventional mode of testing is cumbersome and time-consuming as well as inaccurate, especially if each test is not preceded by introduction of a fresh sample into the aforementioned clearance. The introduction of a fresh sample is necessary for accuracy because the viscosity of a material which has been subjected to shearing stresses of first magnitude is not identical with the viscosity of the same material during subsequent application of shearing stresses of a different second magnitude. Nevertheless, many presently known viscometers are designed in such a way that a full series of tests is carried out with a single sample whereby the mobile measuring element moves from its starting position at a first speed during a first testing stage (when it is connected with a relatively small weight), from the same starting position but at a higher second speed during the next testing stage (as a result of connection of a heavier second weight), and so forth. Thus, the just described testing methods are carried out by utilizing one and the same sample in spite of the fact that such measurements are not entirely accurate due to the above-outlined properties of viscous materials (namely, that the viscosity changes in response to the application of shear stresses of different magnitude, one after the other, always to the same sample). The just mentioned characteristic of a viscous material is especially pronounced if the sample is subjected to different shear stresses at rapidly following intervals. The introduction of a fresh sample after each testing step is avoided because this would unduly prolong the duration of a complete test. Each individual testing step can take more than one minute. Moreover, and especially if the material to be tested is a highly viscous liquid or a paint (e.g., ink) which adheres to the stationary and mobile measuring elements, each introduction of a fresh sample must be preceded by lengthy, thorough and tedious cleaning of both measuring elements. Such situation will arise when the viscometer is used to ascertain the rheological properties of a series of inks, oil paints or the like.